Various methods for preparing isocyanates have been reported in the literature. An important commercial method for preparing isocyanates involves the phosgenation of primary amines. Isocyanates also can be prepared utilizing the Curtius rearrangement of an acid azide in a neutral solvent. The Curtius reaction is used primarily for the preparation of short chain aliphatic diisocyanates and unsaturated isocyanates. The Hofmann rearrangement of amides to form isocyanates is useful for preparing isocyanates which do not react with water since an aqueous medium is required. Another, less frequently used reaction, is the Lossen rearrangement of hydroxamic acids. The process can be exemplified by the preparation of octamethylene diisocyanate from the disodium salt of sebacic dihydroxamic acid. Other procedures for preparing isocyanate compounds are described in Saunders and Frisch: Polyurethanes:Chemistry and Technology I. Chemistry, Part I, pages 17-28, Interscience Publishers, New York, N.Y., 1962.
Considerable quantities of relatively high molecular weight, cross-linked, by-products are formed in these processes, particularly in the phosgene process. Generally, the by-products are obtained in the form of a tar-like, non-distillable residue during and after the distillation of the crude isocyanate concentrate solutions obtained in the phosgenation of amines.
In a commercial method for preparing diisocyanates, an amine solution is mixed with phosgene at a low or moderate temperature. The resulting slurry is then treated with additional phosgene at a higher temperature such as from about 120.degree.-150.degree. C., and the product is obtained by distillation. During the distillation of the concentrate, relatively high molecular weight insoluble products containing uretdione, isocyanurate, carbodimide, uretone imine, urea and biuret groups are formed under the conditions of the distillation. The residue which remains after distillation are slag-like materials which are generally substantially insoluble in conventional solvents, and although considerable effort has been expended in developing uses for the residues, a vast majority of the distillation residues are either dumped or burned in furnaces with considerable difficulty. In those instances where the distillation residues have been burned, the deposits of firmly adhering, substantially non-flammable tarry masses often accumulate in the combustion chamber, and some of these tarry masses have decomposed explosively at temperatures above about 500.degree. C.
U.S. Pat. No. 4,251,401 describes the preparation of stable suspensions of substantially monomer-free, insoluble, powdered tolylene diisocyanate distillation residues in polyhydroxy compounds. The suspensions are reported as being useful as the polyol component in the production of polyurethane plastics including foamed polyurethane plastics. U.S. Pat. No. 4,297,456 describes the process for working up the distillation residue obtained in the commercial production of tolylene diisocyanate by grinding, and optionally, accompanied and/or followed by chemical modification reactions. The finely divided powder is reported as being useful as a reactant filler in the production of a variety of plastics.
Other processes have been proposed for handling and disposing isocyanate distillation residues. In Col. 1 of U.S. Pat. No. 4,297,456, a number of patents and publications are described which relate to dissolving TDI distillation residues.